Method for controlling the temperature in a kiln

ABSTRACT

A method and apparatus for controlling the temperature in a kiln in which cement clinker is manufactured. The temperature is controlled by controlling the fuel rate to the kiln. The fuel rate is adjusted according to a calculation that is made of the sulphur evaporation in the kiln, thereby obtaining a measure of the instantaneous temperature in the burning zone. The evaporation factor is calculated on the basis of measurements of the sulphur content in the cyclone material which passes to the kiln or a measurement of the sulphur content in the exit gases out of the kiln.

BACKGROUND OF THE INVENTION

The present invention relates to a method for controlling thetemperature in a kiln for manufacturing cement clinker.

It is common knowledge that cement is manufactured by a number of rawmaterials, particularly lime (CaCO₃), clay, sand, pyrite ash, fly ashand other materials being mixed and interground into a raw meal in whichthe content of the oxides CaO, SiO₂, Al₂ O₃, Fe₂ O₃ must be specificallyadapted within relatively narrow boundaries. The raw meal issubsequently preheated and calcined, during which process H₂ O and CO₂are driven off. A range of reactions will then occur between the oxides,first and foremost the following reactions:

2 CaO+SiO₂ →(CaO)₂ (SiO₂) (belite) (CaO)₂ (SiO₂)+CaO→(CaO)₃ (SiO₂)(alite)

These reactions between solid substances take place in a molten mass,with the aluminium and iron oxides which are necessary for forming themolten mass.

For every burning process the overall objective is always to ensure thatthe sufficient amount of alite is formed and that, simultaneously, thevolume of free, not yet consumed CaO is reduced to an acceptably lowlevel. Traditionally the burning process takes place in a rotary kilnand the final reaction occurs in the burning zone of the rotary kiln,with the reaction process being controlled by a regulation of thetemperature by adjusting the firing rate and the flame setting. Thetemperature during the burning process approaches the 1400°-1450° C.range for the common types of clinker.

Accordingly, it will be necessary to perform a measurement providingcontinuous indication of the temperature in the kiln, hence indicatingalso the composition and quality of the clinker, but so far it has notbeen possible to measure the interesting temperature directly.

Thermocouples (Pt-PtRh) are impossible to install in a manner ensuringthat they are not destroyed within a short period of time as a result ofthe contact with the hard clinker.

Radiation pyrometers can be used, but only if there is good visibilityin the burning zone, which is rarely the case since a certain dust loadwill inevitably be generated during the burning process.

An indirect signal, which is extensively used to indicate thetemperature, is a measurement of the force which is used to rotate thekiln. The reason why this signal can be used is that the higher thetemperature of the clinker, the greater the amount of molten mass beingformed, hence causing more of the charge to be drawn higher upwardsalong the side of the rotary kiln during rotation. As a result, themoment of force (force x arm) will be increased, hence increasing alsothe power required to rotate the kiln. However, the moment of force is arelative signal which is affected by a multitude of factors: arbitraryskewness in the crust formation, the adhesive properties of the rawmaterials along the entire length of kiln etc. Consequently, it isimpossible to indicate exactly what the moment should be to ensuresufficient burning.

Another method for measuring the temperature in the burning zoneinvolves measurement of the NO_(x) -emission from the kiln. The NO_(x)formation in the burning zone relates specifically to the temperaturelevel in the flame, and it is influenced, at constant production andunaltered burner setting, only by the surplus air required for theburning process, and, since the overall aim is to keep the surplus airconstant, the NO_(x) emission is a direct measure of the burningtemperature. As it is, kilns have been operating for many years on thebasis of NO_(x) -measurements, being controlled both manually andautomatically, e.g. by means of Fuzzy logic.

However, it is a recognized fact that the emission of NO_(x) isdetrimental to the environment, and, therefore, many efforts are mainlyconcentrated on reducing the emission of NO_(x), including the emissionof the rotary kiln, of a cement kiln plant.

These measures severely reduce the possibilities of controlling thekiln. This is best understood by considering a curve of the NO_(x)formation as a function of the temperature T (° C.) (see FIG. 1). Thecurve is found by measuring the NO_(x) formation as a function of thefinal temperature of the clinker subject to a specific flame setting.

When burning common clinker, one has an operating point which issituated in much the same position as A. Here the formed NO_(x) isessentially of thermal origin, i.e. the nitrogen atom in NO_(x)originates from the N₂ of the air, and high/low deviations from thetarget temperature are markedly reflected in a significant change in theamount of formed NO_(x).

When measures are introduced to reduce the NO_(x) level, correspondingto a lower temperature in the kiln, one will approach the operatingpoint B, where the formed NO_(x) originates mainly from the fuel. In theproximity of point B, temperature dependence of the NO_(x) formation isnegligible and, in actual practice, the NO_(x) measurement cannot beused as a control parameter in such a low-NO_(x) operating mode.

In addition to lowering the NO_(x) emission, energy savings are alsoachieved when the flame temperature is lowered, which makes theseconditions particularly desirable during the operation of the kiln.

In order to obtain operating conditions with a low degree of NO_(x)emission, the option of extending the flame in the kiln may be used, forexample by reducing the primary airflow or primary velocity of theburner. By this method the clinker will have a lower final temperature,but, on the other hand, the clinker requires a longer retention time attemperatures above the minimum limit where the alite formation may takeplace.

Another method for obtaining operating conditions with a low degree ofNO_(x) emission and a lower necessary flame temperature involvesaddition of a mineralizer to the raw meal, thereby lowering thetemperature required for the alite formation to take place. Addition ofsulphur and fluoride involves, for example, that the clinker burningprocess may occur at a temperature which is approximately 125° lowerthan normal, i.e. at 1275°-1325° C.

SUMMARY OF THE INVENTION

So, the objective of the invention is to ensure capability ofcontrolling the temperature in the kiln, and hence the manufacture ofthe clinker which is produced in the cement kiln, achieving,simultaneously, a reduction of the NO_(x) emission from kiln to anabsolute minimum.

This is obtained by a method according to the invention where thesulphur evaporation in the kiln is calculated, thereby obtaining ameasure of the instantaneous temperature in the burning zone.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in further details in the following withreference being made to the drawings, where

FIG. 1 shows the amount of formed NO_(x) as a function of thetemperature,

FIG. 2 shows the evaporation factor E as a function of the temperature,

FIG. 3 shows an elementary sketch of the mass flow in a plant formanufacturing cement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Here, evaporation is used to designate the decomposition reactions whichbasically occur at temperatures above 1100° C.:

CaSO₄ →CaO+SO₂ +1/2 O₂ K₂ SO₄ →K₂ O+SO₂ +1/2O₂

The evaporation factor E is defined as the fraction of the sulphur Swhich is fed to the burning zone of the kiln together with the calcinedraw materials, and which is subject to evaporation. This factor is veryinteresting because it increases substantially when the temperature islying within the range 1100°-1500° C. (see FIG. 2) and because this verytemperature range is used for the clinker burning process.

An expression of the evaporation factor E can be found by making twomass balances for a plant for manufacturing cement clinker (see FIG. 3).

Such a plant consists of a system I where the raw materials are beingprepared for burning, and a system II in which burning is taking place.System I may advantageously incorporate a cyclone preheater and possiblya calciner, and system II may advantageously incorporate a rotary kiln.

The sulphur content of the different flows, i.e. the amount of sulphurS_(x) that passes in and out of the two systems I and II, can bemeasured as kg sulphur/hour or as kg sulphur/kg clinker produced at theplant.

Raw materials 1 with a sulphur content S_(feed) and exit gases 2 fromthe rotary kiln with a sulphur content S_(kiln) gas gas are fed tosystem I.

Sulphur, S_(sec).fuel, may also be supplied at 3, via secondary fuel tocalciner or riser duct.

A flow of cooled exit gases 4 with a sulphur content S_(exit) gas isdischarged from system I, and the precalcined or merely preheatedmaterial flows at 5, normally from a cyclone, down into the kiln withthe sulphur content S_(cyc).

The input flows to system II consist of the preheated or precalcinedmaterial S_(cyc) 5 and kiln fuel 6, i.e. primary fuel, with the sulphurcontent S_(prim).fuel. The output flows consist of the exit gasesS_(kiln) gas 2 from the kiln and of the finished clinker 7 with thesulphur content S_(clink).

The evaporation factor E represents, at any particular time, the ratiobetween the amount of sulphur which evaporates in the burning zone andthe amount of solid, combined sulphur which passes into the burningzone. However, the possibility of measuring the sulphur content of thesetwo interesting flows in the burning zone does not exist.

Still, the situation will be such that the amount of sulphur whichevaporates in the burning zone will approximately be equal to the amountof sulphur that passes out of the kiln at 2, S_(kiln) gas, minus thecontribution coming from the primary fuel, S_(prim).fuel, and the amountof solid, combined sulphur which passes into the burning zone willapproximately be equal to the amount which passes down from system I at5, S_(cyc). This involves that: ##EQU1##

The sulphur content in the primary fuel will be constant for a specifictype of fuel and, therefore, the S_(prim).fuel is known if the fuelconsumption during the time period t is known. S_(cyc) will vary withthe time period because of fluctuations in the sulphur input to theburning zone and variations in the temperature in the burning zone. In arotary kiln where the passage of the material from the inlet of the kilnwhere S_(cyc) is measured and to the burning zone may take a relativelylong time, one has to apply a time delay τ to the measurements, inrelation to the measurement of S_(kiln) gas, which happens almostinstantaneously, which means that: ##EQU2##

At any particular point in time, the amount of sulphur down into kiln at5 can be said to be roughly equal to the sum of the amount of sulphurwhich passes in at 7 (S_(feed)), 2 (S_(kiln) gas) and at 3(S_(sec).fuel), but minus the amount of sulphur which passes out at 4(S_(exit) gas). Still, in most kiln systems, the sulphur content in thelast-mentioned stream, S_(exit) gas, will be equal to 0:

S_(cyc) =S_(feed) (t)+S_(kiln) gas (t)+S_(sec).fuel (t) or S_(kiln) gas=S_(cyc) (t)-S_(feed) (t)-S_(sec).fuel (t)

This entails that E(t) can be calculated both by measuring the sulphurcontent in the exit gas from the kiln and by measuring the sulphurcontent in the material being fed to kiln: ##EQU3##

These two expressions will only be valid when the kiln is a rotary kilnbut similar expressions can be developed for other kilns, e.g.stationary kilns.

Generally, there is no point in fitting an SO₂ -meter in the exit gasduct from the rotary kiln. This is because the emission of the kilnsystem into the atmosphere of this detrimental gas component is not inany way related to the measured value of S_(kiln) gas due to the almost100% absorption efficiency of SO₂ in the lower preheater stage or in thecalciner where a relatively large air surplus exists.

Generally, it is uncomplicated to assess the contributions which the rawmaterials and fuel give to the amount of sulphur on the basis of thecurrent analyses and the dosage of feed input.

E(t) can then be calculated, either on the basis of (A) based onmeasurements of the SO₂ content in the kiln exit gas, S_(kiln) gas, orbased on (B) if the sulphur content is measured in the stream whichpasses from the separation cyclone after the calciner and down into thekiln, S_(cyc).

Since SO₂ is the only sulphureous component at a high temperature andair surplus, the easiest method for measuring S_(kiln) gas is to installan SO₂ -meter, which continuously analyzes the kiln exit gas, in thekiln outlet.

A major source of error associated with this method is that calcined rawmeal is whirled up in the exit gas so that the sulphur dioxide ischemically combined through the reaction:

(C) SO₂ +1/2O₂ +CaO→CaSO₄

If the exit gas sample is cleaned and cooled with water, which is thebasic operating principle in certain systems, a part of the SO₂ volumemay also be bound by the water which is alkaline because of the CaO. Asa result hereof, the gas analyzer signal will be too small.

However, if due attention is given to these sources of error, it willoften be possible to assume that a fixed fraction of the SO₂ volume willdisappear, and, accordingly, the signal can still be used forcontrolling the temperature since the real amount of evaporated SO₂ isproportional to the measured amount of SO₂, S_(kiln) gas. measured=constant×S_(kiln) gas.

The method cannot be used for control purposes, if the SO₂ stream out ofthe kiln (e.g. due to a bypass duct being established to reduce SO₂ andthe chloride circulation in the kiln system) is of a size which is solimited that approximately all the SO₂ volume is absorbed by whirled-upCaO. In case of SO₂ presence here, this is more an expression of an airdeficit in the kiln, cf. equation (C), than of a high burning zonetemperature.

The amount of sulphur being fed to the rotary kiln can be determined byseveral known methods. For example by means of an Outokumpo x-rayanalyzer capable of continuously determining the content of the elementsFe, Ca and S in the cyclone material. For this purpose, a substream ofraw meal is extracted from the cyclone, cooled down and compacted beforeit is fed to the analyzer. The signal which is received for theS-content or the S/Ca ratio provides a precise indication of the amountof sulphur that passes down into the rotary kiln.

I claim:
 1. A method for controlling the temperature in a kiln formanufacturing cement clinker wherein raw materials are first preheatedin a preheater by exhaust gas from the kiln, and thereafter fed into thekiln, and further wherein sulfur circulates between the kiln and thepreheater in the form of sulfur dioxide in the exhaust gas from the kilnto the preheater where it is absorbed or adsorbed on basic raw feedmaterial in the preheater and returned to the kiln in a solid state withthe raw material, said method further comprising measuring thetemperature in the kiln by measuring relative to time (t) the amount ofsulfur (S_(feed)) in the exhaust gas from the kiln or the amount ofsulfur (S_(cyc)) in the raw material fed to the kiln so as to obtain anevaporation factor E, the evaporation factor E being the fraction of thesulfur (S_(cyc)) that is converted in the kiln to sulfur (S_(feed)),said evaporation factor E then being adjusted and maintained within aset range by adjusting the amount of fuel to the kiln.
 2. A methodaccording to claim 1, further comprising measuring the sulphur contentS_(kiln) gas (t) in the exit gas of the kiln.
 3. A method according toclaim 1, further comprising measuring the sulphur content S_(cyc) (t) inthe feed stream of material to the kiln.
 4. A method according to anyone of claim 1, 2 or 3, wherein the temperature in the burning zone ofthe kiln is within the range 1100°-1500° C.
 5. A method according to anyone of claims 1, 2 or 3, wherein the temperature in the burning zone ofthe kiln is within the range 1100°-1350° C.
 6. A method according to anyone of claims 1, 2 or 3, wherein the temperature in the burning zone ofthe kiln is within the range 1275°-1325° C.
 7. A plant for manufacturingcement clinker comprising a preheater for preheating cement raw meal,optionally a calciner for calcining the preheated raw meal, and a kilnfor burning the preheated and optionally calcined raw meal, and whereinthe temperature in the kiln is adjusted by varying the fuel rate to thekiln and wherein a meter is fitted between the kiln and the preheaterfor measuring relative to time (t) the amount of sulfur (S_(feed)) inthe exhaust gas from the kiln or the amount of sulfur (S_(cyc)) in theraw material fed to the kiln so as to obtain an evaporation factor E,the evaporation factor E being the fraction of the sulfur (S_(cyc)) thatis converted in the kiln to sulfur (S_(feed)), said evaporation factor Ethen being adjusted and maintained within a set range by adjusting theamount of fuel to the kiln.
 8. A plant according to claim 7 wherein themeter is located in the exit gas duct of the kiln.
 9. A plant accordingto claim 7, wherein the meter is located in the feed stream duct tokiln.